1. Field of the Invention
The present invention relates to telephone systems and more particularly to a line shelf arrangement having pluggable cards providing common logic, a line shelf processor and a plurality of line units, wherein the various cards used in the line shelf have programmable read-only memory for storing specific information concerning the card on which it is mounted.
2. Description of the Prior Art
Telephone systems utilize line shelves as an interface between higher-speed PCM transmission facilities and two-wire or four-wire subscriber lines. In a typical configuration, a line shelf can accommodate up to 48 individual line units, with each unit servicing up to two subscribers.
With improved technology, telephone companies have been offering enhanced telephone service to various types of subscribers. In most cases, the services require unique and different types of line units. In many instances, the line units have substantially similar circuit construction but require different software to perform the designated service. In other instances, similar types of units require different gain or equalization adjustments depending upon the particular installation and the subscriber's needs.
Thus, telephone operating companies must have an inventory of many different types of line units for the various services offered, with different software, gain and equalization adjustments being required on each of the different types of units. The inventory problem is clearly becoming unmanageable.
It is possible to inventory units without software or gain and equalization adjustments, but then the installation requirements become cumbersome, in that for each newly-installed or replaced unit, the appropriate software must be loaded into the unit depending upon the particular type of unit and the service being rendered. The gain and equalization requirements for the subscriber circuit must be adjusted in the field, and this is extremely difficult.
In four-wire type units, the components used therein, particularly transformers, do not have very close tolerances. Wide variations in transformer loss are experienced. Accordingly, gain and equalization adjustments are performed during factory testing. Typically, a completed unit is inserted into a tester and a specific gain resistor is selected and manually inserted into the unit. In like manner, equalization is also adjusted. Such procedures significantly increased manufacturing costs of line units.
With the advent of many different types of telecommunications service, various signaling schemes have been devised which are not compatible with each other. Various signaling schemes use what is commonly known in the industry as A, AB or ABCD type signaling. Signaling translation is required between equipment using different signaling schemes. Special signaling translation circuits for each different type of service were developed. Thus, the free interchange of line units is restricted, in that various types of line units are specifically provided for each service to be compatible with the signaling of the transmission facility. As an example, if ten different types of line units existed to provide ten different types of service and were to be used with transmission facilities of five different vendors, it is possible that 50 different types of line units would have to be manufactured to provide full service for all vendor service combinations. Again, this created an inventory nightmare.
Thus, it was apparent that improvements were required to reduce manufacturing costs, the inventory burden and the manpower required for installing and replacing line units.